The dense root of the Iceland crust

Earth and Planetary Science Letters 206 (2003) 427^440 www.elsevier.com/locate/epsl

The dense root of the Iceland crust Ł lafur Gudmundsson
O Danish Lithosphere Centre, Geocentre Copenhagen, ster Voldgade 10 L, 1350 Copenhagen K, Denmark Received 5 July 2002; received in revised form 22 November 2002; accepted 25 November 2002

Abstract Bathymetry and topography in the North Atlantic Ocean around Iceland are compared to estimates of crustal thickness in the area. Iceland lies much lower than expected based on crustal thickness. This suggests an anomalously low density contrast between crust and mantle beneath Iceland [W. Menke, Geophys. Res. Lett. 26 (1999) 1215^1218]. The relationship between bathymetry and depth to Moho along ridges adjacent to Iceland suggests a normal density contrast there. Continuity of this relationship leads to the conclusion that most of the change occurs in the crust, i.e. the abnormally low density contrast between crust and mantle within Iceland is primarily due to a heavy crust, not light mantle. Gravity modeling also requires the average density of the crust to be unusually high. I argue that this is in fact not abnormal. The lower crust in Iceland is denser because of a higher degree and/or depth of melting beneath Iceland than at adjacent ridges, because of phase transformations occurring within the thick crust and possibly due to fractionation processes in the crust. 8 2002 Elsevier Science B.V. All rights reserved. Keywords: Iceland; density; isostasy; gravity; melting; phase transformations

1. Introduction Recent seismic studies in Iceland have revealed a very thick crust. Three independent types of seismic constraints from wide-angle Moho re£ections, receiver functions, and surface-wave dispersion, provide consistent evidence of a crustal thickness of about 40 km where it is thickest in central Iceland [2^4]. In the meantime Iceland has received considerable attention as a potential example of a ridge-centered plume. Ito and cowork-

* Tel.: +45-3814-2651; Fax: +45-3311-0878. E-mail address: [email protected] (O. Gudmundsson).

ers have modeled the interaction of an active plume upwelling and passive spreading [5,6]. By comparing their model results with ¢eld observations in and around Iceland they conclude that, if the mantle is anhydrous, a relatively broad plume conduit underlies Iceland, about 200 km in radius. If, on the other hand, the mantle contains a signi¢cant amount of water, a much more vigorous and narrower plume conduit is required. The existence of a plume conduit is in general agreement with the low-velocity anomaly imaged by [4,7] beneath Iceland at depths between 200 and 400 km. The bathymetric anomaly around Iceland and the Bouguer gravity anomaly centered on Iceland are reasonably matched along the ridge axis with crustal thickness contributing about 70% of the mass de¢cit to explain the elevation of Iceland

0012-821X / 02 / $ ^ see front matter 8 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0012-821X(02)01110-X

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and the associated gravity anomaly, while the remaining 30% are due to low densities in the mantle (high temperature and depletion). However, the crust is only 33 km thick in their models [5,6] where it is thickest. Menke [1] noted that the topographic level of Iceland is remarkably low compared to the great crustal thickness. He correlated topography with estimates of the depth to the base of the crust within Iceland and demonstrated through an isostatic argument that the density contrast between the crust and mantle beneath Iceland is very low (90 M 10 kg/m3 ). This is about one third of what is expected for basaltic crust sitting on peridotite mantle. Menke [1] argued that since seismic velocities in the lower crust are not anomalously high, and thus the density is not expected to be high there, the density of the mantle beneath Iceland must be extremely low, as low as 3150 M 60 kg/m3 . However, lower-crustal seismic velocities are not well constrained and it is not clear what velocity^ density systematics to expect in the lower crust. Kaban and coworkers [8] presented an analysis of the gravity ¢eld in and around Iceland compiled by Eysteinsson and Gunnarsson [9]. They constructed a model of crustal thickness by correlating bathymetry, corrected for a half-space cooling thermal model, with estimates of crustal thickness. A linear relationship between the two suggests that crustal thickness may be modeled as a simple scaling of corrected bathymetry. They inferred that the lower crust, or seismic layer 4, in Iceland must be very dense in order to satisfy the gravity. The average density in seismic layer 4 (over depth) was estimated in the range 3050^ 3150 kg/m3 , densest where the crust is thickest. On this basis they interpreted layer 4 as a transition between crust and mantle, i.e. a transition between ma¢c and ultrama¢c composition. The crustal thickness model of [8] may not be accurate in the oceans around Iceland because the correlation with bathymetry is only constrained by data within Iceland and the thermal model used is possibly not appropriate there. Nevertheless, they modeled crustal thickness in the ocean to be about 10 km, which is comparable to those estimates of crustal thickness that we have from the area. The potential discrepancy is small compared

to the 30 km change in crustal thickness across Iceland. Thus, their inferred densities for layer 4 are robust. Note that Kaban et al. [8] provide estimates of depth to the 1200‡C isotherm based on residual gravity anomalies after correction for crustal structure. Their results (their ¢gure 20) re£ect density anomalies in the mantle given a model for the crust. There is no signature in those density (thermal) anomalies that can be associated with a plume. There is no broad density anomaly centered on central Iceland as is expected based on the seismic results of [4,7]. If we add a lowdensity anomaly to the mantle and still satisfy the gravity ¢eld, we must increase the density estimates of [7] for layer 4 accordingly. Heller and Marquart [10] examined the admittance of the geoid over bathymetry/topography in the region around Iceland. They found an apparent compensation depth of 30^40 km within Iceland, which suggests at least partial compensation of the topographic anomaly in the mantle beneath the 10^40 km thick crust. From the above it seems clear that the density contrast between the average lower crust and mantle is anomalously low in Iceland. The lowdensity mantle suggested by [1] does not satisfy the gravity and we must infer a very dense root of the Iceland crust as Kaban et al. [8] do. However, their lower-crustal density does not leave room for a low-density anomaly in the mantle, which is required by the admittance of geoid over topography [10], which we expect on the basis of seismic tomography results (e.g. [7]) and is a signi¢cant source of buoyancy in geodynamic models (e.g. [6]). Therefore, the estimates of [8] are potential underestimates of the density of the lower crust in Iceland. I revisit the correlation of elevation and depth to the base of the crust in a manner similar to Menke [1]. I include additional data from the ocean around Iceland and restrict the correlation to estimates of crustal thickness adjacent to the rifts in order to avoid complications due to variable sediment thickness (ocean) and complex cooling history (Iceland). I allow for an isostatic balance with continuous, lateral density variations and examine the conclusions that can be drawn from isostatic arguments critically. I then use

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gravity modeling together with isostatic considerations to suggest a density model for the Iceland hot-spot region.

2. Data During the past decade a number of extensive ¢eld campaigns have been conducted in and around Iceland to study crustal and upper-mantle structure. This has led to a signi¢cantly clari¢ed picture of crustal thickness variations as well as a marked change in the overall thickness estimate within Iceland. Previously, layer 4 of Pa¤lmason [11], at a depth of approximately 10 km [12], was interpreted as anomalous mantle. Bjarnason et al. [13], showed that this layer 4 has a clear base, at about 20 km depth beneath SW Iceland, that is di⁄cult to interpret as anything other than a compositional boundary between ma¢c and ultrama¢c rocks, i.e. Moho. Subsequently, further observations of PmP re£ections along wide-angle pro¢les, e.g. FIRE and ICEMELT [2,14], and a number of receiver-function analyses at broadband seismographs in Iceland, e.g. [3,15], have revealed crust up to 40 km thick in central Iceland. This is corroborated by evidence from shortperiod Love-wave dispersion [4]. It is clear from crustal-thickness estimates to date that Moho depth varies from about 20 km in the south to 40 km in central Iceland and back to about 20 km along the north coast, all over a scale of approximately 300 km. At sea the set of constraints on crustal thickness is also compiling. There is evidence that slightly anomalously thick oceanic crust (10 km) persists to a considerable distance away from Iceland along the Reykjanes ridge [16,17], while to the north of Iceland crustal thickness is found to vary between 6 and 9 km in thickness over a small scale on the Kolbeinsey Ridge [18]. Wideangle pro¢les along the Greenland^Iceland^Faroe (GIF) ridge show approximately 30 km thick crust. The transition from highly anomalously thick crust along the GIF ridge to slightly anomalously thick crust in the adjacent basins is generally not well known. The only detailed seismic informa-

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tion comes from the RISE pro¢le along Reykjanes and the northeasternmost Reykjanes ridge [19]. The crustal thickness reduces smoothly from about 20 km in SW Iceland to 10 km about 100 km SW of Iceland. Smallwood and coworkers [16] and Menke [1] compiled estimates of crustal thickness in the North Atlantic around Iceland. I have supplemented their compilations with a number of estimates from the oceans adjacent to Iceland. The most notable additions are due to Kodaira and coworkers [18,20] from the Iceland Plateau and the Kolbeinsey Ridge, the SIGMA pro¢les across the east Greenland margins [21] and the FIRE pro¢le along the Iceland^Faroe Ridge [22]. I include only those estimates that fall within the 1200 km by 1100 km area centered on Iceland over which the compilations of [9] of gravity, magnetics and bathymetry extend (see Fig. 1). Some of the published receiver-function information is excluded where deemed uncertain. The crustal thickness estimates and their coordinates are listed in Table 1 together with crustal age and source of data. Source of data is further de¢ned in Table 2 [1^3,13^16,18^23]. The estimates come from wide-angle Moho re£ections both on land and at sea and from receiver-function analyses at broadband seismographs on land. I assign a 10% uncertainty to all the estimates. To avoid complications due to a complex and poorly known age distribution in Iceland and variable sediment thickness at sea I concentrate on estimates of crustal thickness adjacent to the rift system. About 30 estimates lie within about 30 km or 3 million years from the rifts. The geographic distribution of the data is shown in Fig. 1. Elevation is estimated at each of the points at which a crustal thickness estimate exists using an average over 30U30 km2 of the bathymetry/topography model of [9]. The bathymetry is isostatically adjusted by removing the weight of the water column at the top and replacing it with mantle material (bm = 3280 kg/ m3 ) at the bottom. I then plot bathymetry/topography against depth to Moho. The results are shown in Fig. 2. The lines through the data are computed by regression and uncertainty represents one standard deviation. The correction for

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70

68

66

64

62

-30

-25

-20

-15

-10

Fig. 1. Measurements of crustal thickness (triangles and circles) in the North Atlantic around Iceland plotted on a background map of bathymetry (500 m contour interval). Only data corresponding to triangles are used here. The solid line shows the pro¢le along which gravity is modeled.

the water layer is perhaps somewhat arbitrary. Rather than removing the water column and letting mantle £ow in to compensate the mass de¢ciency thus created one could choose to compress the water column to form material of crustal density. The e¡ect is to reduce the slope at the left in Fig. 1 by about as much as its uncertainty. I prefer the former choice because crustal structure is not arti¢cially changed. It is clear from Fig. 2 that the relationship between bathymetry and topography and depth to Moho can be divided into two regimes. One I call ‘Iceland’, where crustal thickness is great and

slope is small (black dots). The other I refer to as ‘adjacent ridges’, where crustal thickness is small and the slope is large (grey dots). The transition is fairly sharp at 15^20 km depth to Moho and elevation 0^300 masl (meters above sea level). The slope within Iceland is very low, even lower than estimated by Menke [1]. This is because he did not recognize the change in slope and included estimates near the Icelandic coast, which fall here in the green regime and thus draw the left end of the red line down. It is important to note that the green set of points contains data from both the Reykjanes Ridge and the Kolbeinsey

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Table 1 Coordinates of estimates of crustal thickness, together with their age and source

V

P

Z

d

Source

V

P

Z

d

Source

63.79 64.48 64.86 65.36 66.17 65.71 65.28 64.49 62.90 64.87 64.56 65.92 66.08 63.89 63.61 63.10 64.05 65.50 64.90 63.01 67.78 61.67 69.95 70.35 69.45 69.60 69.00 67.32 65.92

319.9 322.1 317.6 319.2 320.1 316.8 315.5 312.6 39.1 319.6 318.4 317.6 316.4 322.2 323.5 324.5 320.2 317.5 316.9 321.2 321.5 327.0 316.1 315.3 311.7 310.3 39.7 332.0 325.0

21.2 23.2 43.3 34.4 25.5 19.4 35.0 25.0 30.0 30.5 37.0 21.0 27.5 15.2 14.0 12.7 30.0 25.5 35.0 10.0 9.5 10.0 9.0 9.0 9.0 6.5 7.0 33.0 29.5

6 5 0 6 8 0 3 22 43 3 2 3 1 0 0 0 3 2 1 12 8 0 0 1 12 15 19 55 16

SIST SIST ICEMELT ICEMELT ICEMELT FIRE FIRE FIRE FIRE DPW DPW DPW DPW RISE RISE RISE Menke Menke Menke SWM SWM CAM Kodaira Kodaira Kodaira Kodaira Kodaira SIGMA SIGMA

64.14 64.54 65.05 65.74 65.87 65.56 65.09 63.64 64.75 66.13 66.54 65.65 63.95 63.81 63.33 63.03 64.30 65.30 65.25 61.93 70.20 61.51 69.45 69.70 69.15 70.00 69.37 66.50 64.24

321.0 316.8 318.4 319.6 317.5 316.1 314.8 310.7 321.3 318.9 318.0 316.9 321.4 323.1 324.0 323.7 319.5 317.3 316.3 323.6 317.5 326.2 316.0 313.8 310.6 310.0 39.1 328.0 331.0

22.3 42.2 41.7 28.3 24.7 27.8 35.0 30.0 25.6 27.5 16.0 20.5 17.0 13.7 13.3 10.1 30.0 31.5 35.0 8.0 9.0 7.8 7.0 9.0 9.0 6.5 6.0 30.0 10.0

0 1 3 9 3 1 6 33 6 10 4 0 1 0 0 3 4 2 1 10 4 3 1 6 15 16 18 29 30

SIST ICEMELT ICEMELT ICEMELT FIRE FIRE FIRE FIRE DPW DPW DPW DPW RISE RISE RISE RISE Menke Menke Menke SWM SWM CAM Kodaira Kodaira Kodaira Kodaira Kodaira SIGMA SIGMA

V is latitude, P longitude, both in degrees. d is crustal age in millions of years determined from the rendition of [24] of Ivarsson’s [25] age map for Iceland. Source is explained in Table 2.

Ridge at both ends of the range in crustal thickness and bathymetry.

below resent locity about

sea level). The upper crust is meant to repthe seismic upper crust, where seismic veincreases rapidly from a surface value of 3 km/s to 6.5 km/s. The thickness of the

3. Isostasy I proceed in a manner similar to Menke [1] to relate the data presented in Fig. 2 to the density structure. I parameterize density as outlined in Fig. 3. I choose sea level as my reference. Note that the e¡ect of the water column has been removed. The density is speci¢ed by a ¢xed value, bs , at the surface. Kaban and coworkers [8] show that an appropriate value for the Bouguer correction density is bs = 2520 kg/m3 . The crust is divided into an upper crust (down to level u below sea level) and a lower crust (down to a level z

Table 2 Further explanation of sources of crustal thickness estimates in the North Atlantic SIST ICEMELT FIRE DPW RISE Menke SWM CAM Kodaira SIGMA

SIST pro¢le [13] ICEMELT pro¢le [2] FIRE pro¢le [14,22] Receiver functions [13] RISE transect [19] Miscellaneous from Menke [1] Miscellaneous from Smallwood et al. [16] CAM pro¢les from Smallwood and White [23] Iceland Plateau, Kolbeinsey Ridge [18,20] SIGMA pro¢les from Holbrook et al. [21]

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height above sea level (km)

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1 slope = 0.030 ± 0.005 0 slope = 0.116 ± 0.012 -1

-2

0

10

20

30

40

50

depth to Moho (km) Fig. 2. Bathymetry/topography along the rifts through Iceland plotted against depth to Moho.

upper crust is variable, generally 3^5 km within Iceland, but its thickness variations do not correlate with crustal thickness [12]. In normal oceanic crust it is approximately 2 km thick [26]. It is reasonable to take the base of the upper crust to be a £at horizon because Iceland stands about 2 km higher than the adjacent ridges. The appropriate depth to this horizon is about 3 km. I take the density at the base of the upper crust to be bu = 2900 kg/m3 and assume a linear density gradient with depth in the upper crust. The lower crust and mantle are parameterized by average densities, bc and bm , respectively. Isostatic balance is then expressed by: b 0 ðh þ uÞ þ b c ðz3uÞ þ b m ðH3zÞ ¼ C

ð1Þ

where b0 = (bs +bu )/2 and H is the level of compensation. C is a constant. If bm and bc are constants and z and h are the only variables we di¡erentiate with respect to z to get: dh=dz ¼ ð b m 3 b c Þ= b 0 ¼ v b = b 0

ð2Þ

The density contrast, vb, can then be estimated from the slope of h(z) in Fig. 2. If bs = 2500 kg/m3 and bu = 2900 kg/m3 , such that bo = 2700 kg/m3 , we arrive at:

dh=dz ¼ 0:030  0:005Dv b ¼ 81  13 kg=m3 within Iceland dh=dz ¼ 0:116  0:012Dv b ¼ 313  31 kg=m3 at adjacent ridges

A density contrast of vbW300 kg/m3 is expected for normal oceanic crust, depending on the assumed thermal structure. I therefore conclude that the density contrast is normal along ridges adjacent to Iceland, but extremely low within Iceland. Either the crust is very dense in Iceland or the mantle very light. The fact that h(z) is continuous at the change in slope places further constraints on the density structure. Rewrite Eq. 1 with bm = bc +vb and isolate bc : b c ¼ ½C3 b 0 ðh þ uÞ3v b ðH3zÞ

ð3Þ

This holds on both sides of the kink in h(z) at z = zk with all parameters the same except vb. Subtract the two equations for opposite sides to get: 3 3 þ bþ c 3 b c ¼ ðv b 3v b ÞðH3zk Þ=ðH3uÞ

ð4Þ

where the + and 3 superscripts refer to the landward and oceanward side of the slope change, respectively. The proportion of the change of

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ρs z

H

ρu

h

u

ρc ρm

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Iceland plume (e.g. bathymetric anomaly) extend far beyond the region from which data are included here [17], and the data in Fig. 2 do allow for a distributed transition in density structure. In that case both bm and bc may change with z and Eq. 2 no longer holds true. Instead it must be rewritten as: dh=dz ¼

Fig. 3. Parameterization used for isostatic modeling. The density is assumed to increase linearly from the surface density bs to a ¢xed density, bu , at a ¢xed depth, u, below sea level.

density contrast that is due to change of average lower-crustal density is described by the ratio Q = (H3z)/(H3u). I infer zk at the change of slope in Fig. 2, zk = 15^20 km. The thickness of the upper crust, re£ected by the parameter u, is a few km, in any case much smaller than zk . The compensation depth must be greater than the deepest root of the crust and probably much deeper. Thus, the ratio Q must be smaller than unity and probably close to unity. Using zk = 17.5 km, u = 3 km, H = 100 km, vb3 = 313 3 kg/m3 and vbþ = 81 kg/m3 , I get: bþ c 3bc = 197 3 kg/m . Thus, the change in density contrast between lower crust and mantle of 313381 = 232 kg/m3 is explained by a change of lower-crustal density of 197 kg/m3 and a change in mantle density of 35 kg/m3 . In other words, the bulk of the change in density contrast from Iceland to adjacent ridges must occur in the crust, not the mantle. Since the density contrast between lower crust and mantle is normal beneath the ridges adjacent to Iceland it is reasonable to assume normal values for both lower-crustal and mantle density there. Using the same superscript convention as 3 and before I infer densities b3 c = 2965 kg/m 3 3 bm = 3280 kg/m . From the information extracted 3 from Fig. 2 I then infer bþ c = 3160 M 50 kg/m and þ 3 bm = 3245 M 45 kg/m including all sources of uncertainty. This is a very high density for the lower crust in Iceland and somewhat higher than inferred by [8], although not signi¢cantly. I have assumed a sharp transition from anomalous Iceland to normal density structure at adjacent ridges. However, the apparent e¡ects of the

v b = b 0 3d b m =dzWðH3zÞ= b 0 3d b c =dzWðz3uÞ= b 0

ð5Þ

The terms involving dbm /dz and dbc /dz can lead to an overestimate of the density contrast at low z and an underestimate at high z when applying Eq. 2. A reduction of bm with z will cause an overestimate of the density contrast because then dbm / dz 6 0. This is particularly e¡ective where z is small and H3z large. An increase of bc with z will cause an underestimate of the density contrast because then dbc /dz s 0. This is particularly e¡ective where z and z3u are both large. We do infer from Fig. 2 that the density contrast between mantle and lower crust decreases towards Iceland. Thus, either mantle density decreases or lowercrustal density increases towards Iceland, or both. The above e¡ects are therefore inevitable to some degree. A gradational change of density structure from Iceland to adjacent ridges can consequently lead to less extreme variations of density. Fig. 4 shows three models that ¢t the data identically. I have assumed the same ¢xed parameters, H = 100 km, u = 3 km and b0 = 2700 kg/m3 , as before. One model (red) has no density variations in the mantle and a relatively moderate increase in lower-crustal density towards the center of Iceland (i.e. where the crust is thickest). The density contrast between lower crust and mantle is 160 kg/m3 where it is smallest or considerably larger than in the discontinuous model discussed above. It is similar to the overall density model of Kaban et al. [8] in that it contains no broad density anomaly in the mantle and lower crustal density increases within Iceland to a similar level. The density of the lower crust increases by 140 kg/ m3 from right to left. It is this positive gradient of bc beyond crustal thickness of 15^20 km that lowers the slope of topography versus Moho

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Fig. 4. Panel b shows average mantle (dashed) and lower-crustal (solid) densities as functions of depth to Moho according to three models (red, green and blue) that satisfy the data shown in panel a. The solid curve through the data in panel a describes the predicted bathymetry/topography. It is identical for all three models.

depth. The second model (green) has a constant gradient of bm against z with the mantle density dropping from 3280 to 3240 kg/m3 from left to right in the ¢gure. This slope requires a lower density contrast compared to the red model, falling to 60 kg/m3 at z = 40 km. This model is similar to the discrete model considered above in its total variation of density. The third model (blue) has no density variation in the lower crust. In order to cause a lowering of the slope of h(z) at z = 15^20 km a positive gradient of bm with z must be introduced. This model has a density contrast that

paradoxically increases towards the center of Iceland from 300 to 389 kg/m3 despite the fact that the slope of h(z) reduces. This demonstrates a potential pitfall in using Eq. 2 to infer a density contrast from a simple isostatic argument. This model is, however, unrealistic and can be ruled out by gravity modeling. It is unrealistic because the modeled mantle densities exceed that of peridodite at ambient conditions and because we expect a lower density beneath the Iceland melting anomaly due to depletion, high temperature and possible melt content.

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I conclude from this simple modeling that, unless mantle density is signi¢cantly and unrealistically higher beneath Iceland than beneath adjacent ridges, the lower crust beneath Iceland must be considerably denser than its counterpart beneath adjacent ridges. Thus, the density contrast between lower crust and mantle must decrease towards Iceland. It need not be so small as I concluded from the discrete model above or as Menke [1] concluded. Two of the models in Fig. 4 can be considered as end-member models (although this is not strictly true). The red model predicts the density variation of the lower crust subject to the constraint that there is no density variation in the mantle. The blue model predicts the average density of the mantle subject to the constraint that there is no density variation in the lower crust. The green model describes what might be expected allowing for a reasonable density reduction in the mantle towards central Iceland due to depletion and high temperature. In order to distinguish between the models I use gravity modeling.

4. Gravity modeling I use the same parameterization of crustal and mantle density in the gravity modeling as for isostasy. The only di¡erence is that for gravity modeling the structure is speci¢ed as a function of spatial position along a pro¢le instead of Moho depth. The modeling is two-dimensional. Thus, the e¡ect of a density anomaly at depth that does not extend to in¢nity in the direction transverse to the pro¢le is overestimated. Synthetic testing of three-dimensional gravity modeling of anomalies similar in shape to the Iceland crustal anomaly indicates that the resulting Bouguer gravity low is overestimated by 5^10% or about 5 mgal. I use the compilation of Eysteinsson and Gunnarsson [9] of a Bouguer anomaly on land combined with free-air-corrected gravity at sea. I adjust the Bouguer density of 2600 kg/m3 used by [9] with a value of 2500 kg/m3 as suggested by [8]. I furthermore reduce the gravity ¢eld by the e¡ect of a 1% density low in the mantle with a cylindrical shape of radius 100 km at depths between 200

435

and 400 km. This gravity low is centered beneath the region of thickest crust in central Iceland and is consistent with the tomographic results of [4,7] showing a 4% and 2% velocity perturbation for shear and compressional velocity, respectively. This is a correction of about 14 mgal to the anomaly of amplitude about 90 mgal. Note that there is probably a distributed thermal anomaly at shallow depths in the mantle beneath Iceland as well as slightly anomalously thick oceanic crust possibly extending some 1000 km SW along the Reykjanes ridge [17]. However, we do not know the nature of this anomaly and it is thus not included in this modeling as a primary input. The pro¢le is selected to run along the rift zones through Iceland and is shown in Fig. 1. The crustal thickness data used in Fig. 2 are used to constrain crustal thickness along the pro¢le. The density structure is then speci¢ed according to the Moho depth and the models as depicted in Fig. 4 as functions of depth to Moho. Fig. 5 shows crustal thickness along the pro¢le and a comparison of the observed gravity anomaly with those predicted based on the three models presented in Fig. 4. It is clear from the ¢gure that the blue model can immediately be ruled out. It overestimates the gravity anomaly by 50%. The green model slightly under¢ts the anomaly’s amplitude. The red model ¢ts the gravity best. Both the width and amplitude are well-¢tted. Allowing for a density gradient in the lower crust while still adhering to the average densities as speci¢ed in Fig. 4 does not change the ¢t to the gravity anomaly signi¢cantly. Small-scale variations in gravity are presumably caused by variations of uppercrustal thickness and density. I do not attempt to model them here. The best-¢tting, red model has no density variation in the mantle and a lower-crustal density that increases from bc = 2980 to 3120 kg/m3 towards central Iceland. The green model has a mantle density decrease from 3280 to 3240 kg/ m3 and a lower-crustal density increase from 2980 to 3180 kg/m3 from adjacent ridges to the center of Iceland. The ¢t to the gravity cannot distinguish between the red model and one that lies about halfway between the red model and the green. I therefore conclude that the combined

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Fig. 5. Fits to the Bouguer gravity anomaly across Iceland along the rift system. (a) Black curve shows the observed gravity anomaly reduced as described in the text. Red, green and blue curves show predicted gravity anomaly according to the models of the same color as shown in Fig. 4. (b) The parameterization of the density structure. Observations of depth to Moho along the pro¢le are shown with error bars.

modeling of gravity and isostasy indicates that lower-crustal density increases smoothly by 140^ 170 kg/m3 from the adjacent ridges to central Iceland and that average mantle density decreases by 0^20 kg/m3 over the same range. It should be noted that the average mantle density is an aver-

age of the mantle above a horizon at 100 km depth assuming that the density variations in the mantle (other than that due to a 1% plume-stemlike anomaly at 200^400 km depth) are concentrated above this horizon. One of the end-member alternatives (red), with

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no density variation in the mantle and a 140 kg/ m3 increase in lower-crustal density towards central Iceland, agrees with the main features of the model of [8]. The other (average of red and green), with a greater increase of lower-crustal density towards Iceland (170 kg/m3 ) and a 20 kg/m3 reduction of average mantle density, is consistent with the ¢ndings of [10] that the topographic anomaly of Iceland is partly compensated within the mantle as well as the dynamic models of [5,6]. A 20 kg/m3 density anomaly distributed over a 60^90 km depth range would support about 600 m of topography weighing 2500 kg/ m3 . I therefore consider the latter alternative a more likely density model for the Iceland region (preferred model).

5. Discussion A density di¡erence of 140^170 kg/m3 between Icelandic and oceanic lower crust may seem unreasonably high. There are, however, a number of processes that could contribute to this e¡ect. Melting models that satisfy crustal thickness constraints and rare-earth element patterns require higher degrees of melting beneath Iceland than ordinary ridges [5] and consequently an olivineenriched crust. This leads to higher density. If crustal fractionation processes are more extensive in the thicker Iceland crust than at adjacent ridges, this di¡erence may be enhanced. If the melting is ‘whet’ at depth, a larger proportion of the melt is generated at greater depth [29], which leads to iron enrichment and higher density. Further Fe enrichment of the Iceland crust due to iron enrichment of its source is possible if Iceland is formed by the interaction of a spreading center and a plume rising from the core^mantle boundary. Phase transformations from plagioclase to garnet have a signi¢cant e¡ect on gabbroic rocks and are expected to come into e¡ect at depths around 25 km [28], i.e. within the thick Iceland crust. It is possible to estimate the density di¡erence between expected primary melts in Iceland compared to that at ridges using modal analysis and density data for end-member minerals. Using

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the empirical relationship between crustal density and initial pressure of melting, P0 , established by Klein and Langmuir [29] I estimate a density difference of 80 kg/m3 between 23 km thick crust at P0 = 4 GPa (average pressure and average degree of melting Pav = 1.64 GPa and Fav = 20%, respectively) and 7 km thick crust at P0 = 2 GPa (Pav = 0.73 GPa and Fav = 11%). These could be representative of Iceland and normal ridges with crustal thickness in Iceland increased to 40 km by an active upwelling. Thus, 40^50% of the estimated density di¡erence can be accounted for by the expected di¡erence in the degree and pressure of melting. If the upper crust can be identi¢ed as extrusives, and assuming that the extrusives have suffered 20% olivine fractionation [27], then a 30 km thick crust would on average be further enriched in olivine by 3.3% if the extrusive layer is 4 km thick. Thus, the density of the lower crust in Iceland may be elevated by another 10^15 kg/m3 . However, MORB extrusives are also fractionated primary melts [30] and there the upper crust represents a larger volumetric proportion of the whole crust than in Iceland. If the melting process is whet at depth then the viscosity change associated with dehydration dictates that the dry part of the melting column is essentially passive [6] and satisfying REE patterns and crustal thickness in Iceland requires a strongly active £ow in the whet part of the melting zone [27]. Thus, the average pressure of melting is signi¢cantly increased, while the average degree of melting drops. This enhances iron content. Mclennan and coworkers [27] estimate a melt composition for the Herdubreid region in central Iceland with 11% FeO and 15% MgO compared to 9.5% FeO and 15% MgO for the P0 = 40 kbar model of [29]. In addition, deep melting increases the CaO/Al2 O3 ratio [31]. This change in melt composition could cause a density di¡erence of about 20 kg/m3 . Jull and Kelemen [28] studied the density e¡ect of the plagioclase to spinel to garnet phase changes on a number of crustal rocks. They found that in gabbroic rocks the phase changes come into e¡ect at pressures around 0.7 GPa at temperatures less than 800‡C and 0.9 GPa at 1000‡C.

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The density increases by approximately 250 kg/m3 between 0.7 and 1.3 GPa at 800‡C, corresponding to depths of 22 and 40 km. Thus, where the crust is thicker than about 22 km the density e¡ect of the phase transformations is felt and where the crust is 40 km thick the e¡ect is to increase the average density of the lower crust by about 65 kg/ m3 . The e¡ect of the phase transformations is less if the lower crust is hotter than 800‡C. Fischer [32] invokes the same e¡ect to explain the time evolution of the density of the roots of ancient mountain chains. By combining all the above e¡ects it is possible to explain a density di¡erence between lower Icelandic crust and normal oceanic lower crust by about 160 kg/m3 , which is within the range of estimates from isostasy and gravity. The bulk of the explanation lies in olivine enrichment due to higher degree and pressure of melting on the one hand and where the crust is thickest by the e¡ects of phase transformations on the other. It may be possible to gain slightly more in density by Fe enrichment due to Fe enrichment of the source. Klein and Langmuir [29] and Korenaga and Kelemen [33] show that fractionation-corrected Fe8 is high around Iceland that may in part be a source feature. It is not clear what seismic velocities would be associated with such dense lower crust. Kelemen and Holbrook [34] (see also ¢gure 6 of [21]) predict a compressional-velocity di¡erence of 300 m/s between the products of passive upwelling starting at P0 = 4 GPa (23 km thick crust) on the one hand and P0 = 2 GPa (8 km thick crust) on the other according to the melting model of [31]. I evaluated from [29] above that that corresponds to a density di¡erence of about 80 kg/m3 . Extrapolating to a density di¡erence of 160 kg/m3 would then correspond to an increase of compressional velocity of 0.6 km/s or about 7.6 km/s compared to 7.0 km/s. But, it is unclear to what extent iron plays a role in elevating the lower-crustal density beyond an 80 kg/m3 anomaly and high Fe content would lower the velocity while increasing the density. It is feasible that a crust as dense as 3150 kg/ m3 might have an in situ compressional velocity less than 7.5 km/s. That does not violate loose seismic constraints, which consist of an estimate

of 7.2 km/s at the base of the crust where it is a little over 20 km thick [13] and an estimate of 7.35 km/s at the base of the crust where it is 35 km thick based on an extrapolation of the velocity gradient of the middle crust [14]. The high lower-crustal density inferred here and the low-density contrast between the depth-averaged lower crust and uppermost mantle raise the question if the lowermost crust in Iceland is not potentially gravitationally unstable as Jull and Kelemen [28] postulate for the continental crust. With a depth-averaged, lower-crustal density of 3150 kg/m3 (preferred model) and a density increase of 250 kg/m3 in the bottom 18 km of a 40 km thick crust [28] the base of the crust will have a density of about 3335 kg/m3 , i.e. denser than the top of the mantle. Therefore, it is possible that some of the thickness variations of the Icelandic crust are reduced with time (distance away from rifts) by delamination as [28] suggested and modeled.

6. Conclusions The density structure of Iceland and the surrounding area has been examined through the use of isostatic arguments based on the variation of elevation with Moho depth. Simplistic arguments that ignore gradational lateral variation of density lead to the conclusion that the lower crust in Iceland is 200 kg/m3 denser than its counterpart beneath adjacent ridges. However, this estimate can be signi¢cantly in£uenced by the e¡ect of lateral density gradients in the lower crust and upper mantle. Taken in isolation, the isostatic argument can paradoxically lead to a model with increasing density contrast between the lower crust and the uppermost mantle in the direction in which the elevation gradient decreases. Taking into account the gravity anomaly centered on Iceland and the expectation that mantle density decreases towards the center of Iceland, i.e. the center of a hot spot, due to depletion in the mantle because of melting and due to high temperatures we are, however, stuck with the conclusion that the lower crust in Iceland must be anomalously dense and that the density contrast between the

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lower crust and mantle is anomalously low (110^ 160 kg/m3 ). A dense lower crust is to be expected in Iceland. The very high densities of about 3120^ 3150 kg/m3 (average lower-crustal density over depth) can be explained by combining the e¡ects of olivine enrichment due to a high degree of average melting, iron enrichment due to deep melting, and plagioclase to spinel to garnet phase transformations within the crust. We do not need to interpret the lower crust as a chemical transition between ma¢c an ultrama¢c rocks to explain the high density of the lower crust.

[8]

[9]

[10]

[11] [12]

Acknowledgements I thank Stefan Bernstein, Ole Stecher, Kresten Breddam and John Hopper for helpful discussions. An earlier version of this manuscript was signi¢cantly improved by the constructive comments of Gudmundur Pa¤lmason, Robert White and an anonymous reviewer. I acknowledge the Danish Research Foundation (Grundforskningsfonden) for support.[SK]

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[14]

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